Sound system and method of sound reproduction

ABSTRACT

A sound reproduction system comprises a left and right speakers located in close proximity, and a sound processor which provides audio signals to the pair of speakers. The sound processor preferably derives a cancellation signal from the difference between the left and right channels. The resulting difference signal is scaled, delayed (if necessary), and spectrally modified before being added to the left channel and, in opposite polarity, to the right channel. The spectral modification to the difference channel preferably takes the form of a low-frequency boost over a specified frequency range, in order to restore the correct timbral balance after the differencing process. Additional phase-compensating all-pass networks may be inserted in the difference channel to correct for any extra phase shift contributed by the spectral modifying circuit. The technique may be used in a surround sound system.

RELATED APPLICATION INFORMATION

This application is a continuation of U.S. patent application Ser. No.12/561,607, filed Sep. 17, 2009 currently pending, which is a divisionalof U.S. patent application Ser. No. 11/762,725, filed on Jun. 13, 2007,now U.S. Pat. No. 7,593,533, which is a continuation of U.S. applicationSer. No. 10/074,604, filed on Feb. 11, 2002, now U.S. Pat. No.7,254,239, issued Aug. 7, 2007, which in turn claims priority to U.S.Provisional Application Ser. No. 60/267,952, filed on Feb. 9, 2001. Theforegoing applications are hereby incorporated by reference as if setforth fully herein.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The field of the present invention relates to sound reproduction and,more specifically, to a speaker configuration and related soundprocessing for use in a sound system.

2) Background

Attaining optimal sound quality in surround sound or multi-channel soundsystems, over the largest possible listening area, can be quitechallenging. Some of the difficulties in achieving optimal sound qualityin such systems result from the fact that a wide variety of differentsurround sound and multi-channel audio formats and speakerconfigurations exist, so that a particular sound system may havereasonably acceptable sound with respect to one or perhaps two audioformats yet sub-optimal sound with respect to other audio formats.Therefore, where a consumer desires, for example, to use a single soundsystem to play sound recordings in a variety of different formats,different levels of sound quality, some of which are poor or impaired,are likely to be experienced. While the user can adjust speakerpositioning or relative balances to try to improve sound quality, suchtechniques may involve significant manual effort or inconvenience, maybe hard to reproduce consistently, and may benefit only one or perhaps afew listeners in a relatively small portion of the listening area.

Existing surround sound recording formats include those referred to as5.1, 6.1 and 7.1. The 5.1 surround format comprises a compressed datastream containing five channels, generally designated left, center,right, surround left, and surround right, named for the speakerpositions for which the channel information is intended. A low frequencyeffects channel is formed by a combination of the five other channels,and may be provided to a sub-woofer. The 6.1 surround format includesthe same five channels as the 5.1 surround format, but adds a surroundback channel, which may be fed to one or more back speakers in asurround sound system. The 7.1 surround format is similar to the 6.1surround format, but has two surround back channels (surround back leftand surround back right) rather than a single back channel, for a totalof seven channels. Thus, the 5.1 surround format has two surroundchannels (surround left and right), the 6.1 surround format has threesurround channels (surround left, right and back), and the 7.1 surroundformat has four surround channels (surround left and right, and surroundback left and right).

Basic surround system speaker configurations generally include from sixto eight speakers placed at conventionally well-established locations,according to the type of surround format they are intended to play. Asix-speaker surround system typically includes left, right and centerspeakers (with the right and left speakers spaced widely apart), asub-woofer, and surround left and right speakers (which may be monopolaror dipolar in nature). A seven-speaker surround system typicallyincludes the same speaker arrangement as the six-speaker surroundsystem, but adds a back surround speaker. An eight-speaker surroundsystem typically includes the same speaker arrangement as thesix-speaker surround system, but adds a back left surround speaker and aback right surround speaker.

The enjoyment experienced by a listener in a surround sound system canbe affected by a number of factors, including the listener's physicalposition relative to the various speakers, as well as by the particularformat of the audio track being played on the system. For example, whena 5.1 surround format soundtrack is played on an eight-speaker (7.1)surround system, certain anomalies may occur. An example is that, if the5.1 surround left and surround right audio signals are monaural, thenthe left and right surround effects can disappear, being replaced by asingle central “phantom” sound image at the rear. Another phenomenon isthat if the listener is positioned in the middle of the surround leftand surround right speakers, he or she may perceive the surround leftand right sound (if monaural) to be higher in volume that it otherwisewould be, primarily due to the additive effect of the sound wavesintersecting at the listener's position (known as a “lift” effect). Ifthe sound pans from one side to the other (e.g., from left to right),the sound volume may appear to increase as left/right balance isachieved, and then appear to decrease as the sound continues to pan,even though the audio output level remains constant, due to the same“lift” effect. The sound quality may also seem to be “unstable,” in thesense that if the listener moves from the center position, the soundmight seem to “flip” from one side to the other.

Some of these effects can be mitigated in 5.1 surround sound systems bythe use of adaptive decorrelation with respect to the surround left andright speakers, which derives two substantially decorrelated signalswhen the surround left and right signals are monaural, in order toprovide an improved enveloping surround effect.

When a 6.1 surround format soundtrack is played on an eight-speaker(7.1) surround system, certain other anomalies may be experienced. Sincethe two rear surround speakers (left and right) are each fed with anidentical monaural signal (that is, the same surround back signal), acentrally located “phantom” image may result when the listener ispositioned approximately equidistant from the speakers. Reported sideeffects of this arrangement include “coloration” associated with thephantom image (for example, the sound may seem “unnatural”), a narrow“sweet spot” due to lack of sound image stability when the listenermoves off center, and a comb filter effect (in other words, nulls may beproduced due to sound wave cancellation effects).

Besides surround systems, a variety of multi-channel recording andplayback systems also exist. Examples of some common multi-channel soundsystems are Dolby AC-3, DTS, and DVD-Audio, each of which has its ownspecific digital encoding format. Unlike cinema sound, there isgenerally no single adopted standard of either loudspeaker type (e.g.,full range, satellite plus sub-woofer, dipole, monopole) or speakerlayout for most multi-channel audio formats. Any user therefore desiringto listen to multi-channel soundtracks, and/or any of the surroundformats (5.1., 6.1 and 7.1), is required either to accept one speakerlayout optimized for a particular audio format and experience acompromised performance for all others, or to reconnect various speakersto suit the audio format a particular soundtrack.

Beyond the surround sound environment, other sound systems also facesimilar challenges, such as attaining a suitably wide “sweet spot” inwhich the perceived area and stability of a stereo sound image ismaximized. In most traditional sound systems, the convention has been toplace left and right speakers far apart physically, under the theorythat the human ear is thereby better able to perceive the richness ofthe audio subject matter. However, under many left/right speakerconfigurations, the sound at off-axis listening positions may besub-optimal. The quality of sound at a given off-axis listening positionmay be affected not only by the difference between left and rightvolumes resulting from the different distances to the left and rightspeakers, but also by the slight difference in time it takes the auralinformation to reach the listener.

Accordingly, it would be advantageous to provide an improved soundsystem which overcomes one or more of the foregoing problems orshortcomings.

SUMMARY OF THE INVENTION

The present invention is generally directed to improved soundreproduction systems and methods involving a speaker configurationand/or placement, and related sound processing, for enlarging theperceived area and stability of a sound image generated from right andleft source signals.

In one aspect, a sound reproduction system comprises a pair of speakers(left and right) located in close proximity, and a sound processor whichprovides audio signals to the pair of speakers. According to a preferredembodiment, the sound processor acts to “spread” the sound imageproduced by the two closely spaced speakers by employing across-cancellation technique wherein a cancellation signal is derived,for example, from the difference between the left and right channels.The resulting difference signal is scaled, delayed (if necessary) andspectrally modified before being added to the left channel and, inopposite polarity, to the right channel. The spectral modification tothe difference channel preferably takes the form of a low-frequencyboost over a specified frequency range, in order to restore the correcttimbral balance after the differencing process which causes a loss ofbass when the low-frequency signals in each channel are similar.Additional phase-compensating all-pass networks may be inserted in thedifference channel to correct for any extra phase shift contributed bythe usually minimum-phase-shift spectral modifying circuit so that thecorrect phase relationship between the canceling signal and the directsignal is maintained over the desired frequency range.

Alternatively, a linear-phase network may be employed to provide thespectral modification to the difference channel, in which casecompensation can be provided by application of an appropriate, andsubstantially identical, frequency-independent delay to both left andright channels.

The various speaker configuration and sound processing embodiments asdescribed herein may be employed in connection with a surround soundsystem to achieve improved sound reproduction. A sound reproductionsystem for a surround sound stereophonic system may comprise a set ofspeakers (e.g., front, left, center, surround left, and surround right),including a pair of surround back speakers located in close proximity,and a sound processor. The sound processor receives left and rightsurround channel signals (either side or rear surround signals), andgenerates a difference signal therefrom. The resulting difference signalmay be processed as described above—i.e., scaled, delayed (if necessary)and spectrally modified before being added to the left channel and, inopposite polarity, to the right channel. Additional phase-compensatingall-pass networks may, as noted above, be inserted in the differencechannel to correct for any extra phase shift contributed by the usuallyminimum-phase-shift spectral modifying circuit so that the correct phaserelationship between the canceling signal and the direct signal ismaintained over the desired frequency range.

Further embodiments, variations and enhancements are also disclosedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating playback of a soundtrack in a 5.1surround system.

FIG. 2 is a diagram illustrating playback of a 5.1 surround formatsoundtrack in a 7.1 surround sound system.

FIG. 3 is a diagram illustrating playback of a 6.1 surround formatsoundtrack in a 7.1 surround sound system.

FIG. 4 is a diagram illustrating the concept of a “sweet spot” in thecontext of 6.1 surround format playback in a 7.1 surround sound system.

FIG. 5 is a diagram illustrating movement of the phantom image inconjunction with the listener's movement.

FIG. 6 is a diagram of a speaker configuration for a surround soundsystem, in accordance with a preferred embodiment as described herein.

FIG. 7 is a diagram illustrating 6.1 surround format playback in thesurround sound system shown in FIG. 6.

FIG. 8 is a simplified block diagram of a sound processing system inaccordance with one or more embodiments as disclosed herein, as may beused, for example, in connection with the speaker configurationillustrated in FIG. 6.

FIG. 9-1 is a more detailed diagram of a sound processing system as maybe used, for example, in connection with the system illustrated in FIG.6

FIG. 9-2 is a diagram of a sound processing system in general accordancewith the layout illustrated in FIG. 9-1, further showing examples ofpossible transfer function characteristics for certain processingblocks.

FIG. 10 is a diagram of a sound processing system illustratingrepresentative transfer functions.

FIG. 11 is a diagram of a sound system in accordance with the generalprinciples of the systems illustrated in FIGS. 8 and 9, as applied inthe context of a surround sound system.

FIG. 12 is a conceptual diagram illustrating processing/operation for5.1 surround format playback in the context of a surround sound systemsuch as shown, for example, in FIG. 6 or 11.

FIGS. 13 and 14 are graphs illustrating examples of frequency responseand phase transfer functions for a sound processing system havingparticular spectral weighting and other characteristics.

FIGS. 15-1, 15-2, and 15-3 are graphs illustrating examples of gainand/or phase transfer functions for a sound processing system inaccordance with FIG. 9-2.

FIG. 16 is a diagram of a sound processor employing a linear spectralweighting filter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to various embodiments as disclosed herein, a preferred soundreproduction system comprises, in one aspect, a pair of speakers locatedin close proximity, and a sound processor which provides audio signalsto the pair of speakers. The sound processor preferably acts to “spread”the sound image produced by the two closely spaced speakers by employinga cross-cancellation technique wherein a cancellation signal is derived,for example, from the difference between the left and right channels.The resulting difference signal is scaled, delayed (if necessary) andspectrally modified before being added to the left channel and, inopposite polarity, to the right channel, thereby enlarging the perceivedarea and stability of the stereo sound image. Further details ofpreferred sound processing techniques are described later herein.

Some advantages of various embodiments disclosed herein can beappreciated by way of contrast and comparison with conventionalsurround/multi-channel sound systems. FIG. 1, for example, is a diagramillustrating playback of a surround-encoded soundtrack in a 5.1 surroundsystem 100. As shown in FIG. 1, the 5.1 surround system 100 includes afront left speaker 104, a front right speaker 105, a center speaker 102,a sub-woofer 109, a surround left speaker 114, and a surround rightspeaker 115. In the example shown in FIG. 1, the surround left and rightspeakers 114, 115 are both dipolar speakers, which distribute sound inmultiple (typically opposite) directions and are thereby provideimproved ambient sound. The surround left and right speakers 114, 115are typically widely spaced on opposite sides of a room (or otherlistening space), at positions which are above and slightly to the rearof the desired listening position.

The speakers 102, 104, 105, 109, 114, and 115 in the 5.1 surround system100 are generally arranged to provide optimum sound for a listener 107positioned in the approximate center of the speaker arrangement.However, a 5.1 surround system lacks an effective directional componentto the immediate left and right sides and to the rear of the listener107. Therefore, a 6.1 or 7.1 surround system, both of which have a rearspeaker component, is generally capable of providing superior sound andaudio effects in certain contexts. A 6.1 surround system, as previouslyindicated, adds a single rear surround speaker, while a 7.1 surroundsystem adds two rear surround speakers typically spaced relatively farapart from one another.

FIG. 2 is a diagram of a 7.1 surround system 200, illustrating playbackof a 5.1 surround-encoded soundtrack. As shown in FIG. 2, the 7.1surround system 200 includes front left and right speakers 204, 205, acenter speaker 202, a sub-woofer 209, a surround left speaker 214, asurround right speaker 215, a surround back left speaker 224, and asurround back right speaker 225. In the particular example of FIG. 2, aswith FIG. 1, the surround left and right speakers 214, 215 are dipolarin nature. The surround back left and right speakers 224, 225 aretypically spaced relatively far apart behind the listener 207. When a5.1 encoded soundtrack is played on a 7.1 surround system 200 such asshown in FIG. 2, the surround left and right speakers 214, 215 receivethe left and right surround channel information, and the surround backleft and right speakers 224, 225 may or may not receive the left andright surround channel information, depending upon how the user hasprogrammed the system 200. In either case, certain anomalies can occur.For example, if the left and right surround channels are monaural, theleft/right surround effect can seem to disappear and be replaced by asingle central “phantom” sound image 230 at the rear of the listener207. This effect can be mitigated by the use of adaptive de-correlation,which involves derivation of two substantially de-correlated signalsfrom the single monaural channel in order to provide an improvedenveloping surround effect.

FIG. 3 is a diagram illustrating 6.1 surround format playback in a 7.1surround system. In FIG. 3, the speakers labeled 3xx generallycorrespond to the same speakers labeled 2xx in FIG. 2. When a soundtrackin a 6.1 surround format is played on a 7.1 surround system 300 such asshown in FIG. 3, the surround back speakers 324, 325 are fed withidentical monaural signals (derived from the single surround backchannel in the 6.1 encoding format), which may or may not be delayedwith respect to each other to compensate for unequal distances from theoptimum listening position. As illustrated in FIG. 3, the identicalmonaural signals being played through the surround back speakers 324,325 produces a central “phantom” sound image 330 when the listener ispositioned approximately equidistant from them. Reported side effectsinclude “coloration” associated with the phantom sound image 330, whichcan lead to listener confusion or an unnatural sound, a narrow “sweetspot” (see FIG. 4) due to lack of sound image stability when thelistener moves off center from the axis which is equidistant from bothsurround back speakers 324, 325 (see FIG. 5), and suppression of certainfrequency ranges due to cancellations (i.e., nulls) caused by a “combfilter” effect as the sound waves interfere with one another. As aresult, the sound quality of a 6.1 surround format soundtrack, whenplayed back in a 7.1 surround system 300, can suffer significantly,particularly for listeners that are not positioned in an optimumlistening position.

As previously indicated in the Background section hereof, replay ofsoundtracks in other multi-channel formats (such as Dolby AC-3, DTS orDVD-Audio) can also suffer from similar effects, depending upon thenature of the signals fed to the different left/right and back surroundspeakers.

FIG. 6 is a diagram showing a speaker configuration for a surround soundsystem 600 in accordance with a preferred embodiment as describedherein. The sound system 600 of FIG. 6 includes, similar to the systems200 and 300 shown in FIGS. 2 and 3, respectively, front left and rightspeakers 604, 605, a front center speaker 602, a sub-woofer 609, asurround left speaker 614, and a surround right speaker 615. The soundsystem 600 further includes a surround back left speaker 624 and asurround back right speaker 625, which are preferably positioned inclose proximity to one another, possibly even within the same speakerenclosure. The surround back left and right speakers 624, 625 arepreferably identical and may be either dipolar or monopolar in nature,but are shown in FIG. 6 as monopolar. The speaker configuration of thesound system 600 illustrated in FIG. 6, coupled with a preferred soundprocessing technique, can provide improved sound quality when, forexample, playing audio tracks recorded in any of the surround sound ormulti-channel formats.

When the sound system 600 of FIG. 6 is used to play a soundtrackrecorded in 7.1 surround format, the various left, right, center, andsurround left/right channel audio signals are fed to the appropriateindividual speakers, as would normally be done with a typical 7.1surround speaker configuration. However, the surround back left andright speakers 624, 625 preferably receive the surround back rightchannel audio signal and surround back left channel audio signal aftersound processing as further described in more detail later herein.

When, on the other hand, the sound system 600 of FIG. 6 is used to playa soundtrack recorded in 6.1 surround format, the various left, right,center, and surround left/right channel audio signals are again fed tothe appropriate individual speakers, as would normally be done with atypical 7.1 surround speaker configuration. Typically, assuming thatSurround EX playback is properly selected (e.g., a Surround EX flag ispresent), the surround back left and right speakers 624, 625 bothreceive and respond directly to the surround rear channel audio signal.The central rear sound image produced by the closely spaced surroundback left and right speakers 624, 625 from the monaural signal (i.e.,the surround rear channel audio signal) is stable over a much widerarea, as compared to widely spread surround back left and rightspeakers, and has significantly less “coloration” or unnaturalness thanthe audio sound produced by such widely spaced rear surround speakers.

In some instances, such as, for example, where the 6.1 Surroundsoundtrack is matrix-encoded, or where Surround EX processing is notinvoked for whatever reason, a somewhat different type of playback maybe experienced. In such a case, the sound system may effectively treatthe soundtrack as a 5.1 soundtrack, and may send to the surround backleft and right speakers 624, 625 the surround left and right channelaudio signals, which may be mixed with at least some portion of themonaural channel information (if the soundtrack is matrix encoded).According to a preferred sound system as disclosed herein, the surroundback left and right speakers 624, 625 both receive and respond directlyto the surround rear channel audio signal, if such information ispresent, and, after suitable sound processing, as further describedherein, to the surround left/right channel audio signals. FIG. 7illustrates the playback of a 6.1 surround-encoded soundtrack in thesound system 600 of FIG. 6 in such a situation. As shown in FIG. 7, awide monaural sound image is projected from the surround back left andright speakers 624, 625. The surround left and right channel audiosignals are fed to both the surround left and right speakers 614, 615,and to the surround back left and right speakers 624, 625 after soundprocessing as further described later herein.

When the sound system 600 of FIG. 6 is used to play a soundtrackrecorded in 5.1 surround format, the various left, right and centerchannel audio signals are fed to the appropriate individual speakers, aswould normally be done with a typical 7.1 surround speakerconfiguration. Preferred operation with respect to the surround left andright speakers 614, 615 and surround back left and right speakers 624,625 depends in part upon the nature of the surround left/right channelaudio signals. When the surround left/right channel audio signals aremonaural in nature, the sound system 600 preferably uses adaptivede-correlation to provide a de-correlated signal for the side surroundspeakers 614, 615, and provides a direct feed to the surround back leftand right speakers 624, 625 to produce a superior rear central image.However, when the surround left/right channel audio signals are stereoin nature, the surround left/right channel audio signals are feddirectly to the surround left and right speakers 614, 615 withoutadaptive de-correlation, and, if desired, after suitable soundprocessing as further described herein, to the surround back left andright speakers 624, 625. The surround left and right channel audiosignals are processed such that the apparent rear sound image size isincreased, and its stability is improved at off-axis listeningpositions. The appropriately apportioned and summed output of the twoside surround speakers 614, 615 and the two surround back speakers 624,625 creates a near-continuous rear-half sound field, thereby improvingthe sound experience for listeners over a wider area.

FIG. 12 is a simplified diagram conceptually illustrating playback of a5.1 surround format soundtrack in the sound system 600 of FIG. 6, whenthe sound system 600 is configured to apply the surround left and rightchannel audio signals 1211, 1212 to the rear surround speakers 1224,1125. As illustrated in FIG. 12, when the surround left and rightchannel audio signals 1211, 1212 are monaural, adaptive de-correlationprocessing (as represented by blocks 1271 and 1272) is activated, andwhen they are stereo in nature, adaptive sound processing for the rearsurround speakers 1224, 1225 (as represented by block 1201) isactivated.

More generally, the techniques described herein are capable of producingpotentially improved sound for a stereo signal in connection with aspeaker configuration that includes two speakers placed in closeproximity. Whenever a stereo signal from any encoded program (e.g.,surround sound or multi-channel soundtrack), or any audio product orsource, is fed directly to the appropriate right and left speakers(e.g., left and right surround speakers) and, after suitable soundprocessing as further described herein, to the pair of speakers placedin close proximity (e.g., surround back speakers). The pair of closelyspaced speakers is thereby capable of generating a sound image ofimproved stability and quality over a wider area, thus enlarging theoptimum listening area and providing greater satisfaction to thelisteners.

Further details regarding preferred sound processing for closely spacedspeakers (such as rear surround speakers 624, 625 in FIG. 6) will now bedescribed. FIG. 8 is a generalized block diagram of a sound processingsystem 800 in accordance with on embodiment as disclosed herein, as maybe used, for example, in connection with the speaker configurationillustrated in FIG. 6, or more generally, in any sound system whichutilizes multiple audio channels to provide stereo source signals. Asshown in FIG. 8, a left audio signal 811 and right audio signal 812 areprovided to a sound processor 810, and then to a pair of closely spacedspeakers 824, 825. The left audio signal 811 and right audio signal 812may also be provided to left and right side (surround or non-surround)speakers, not shown in FIG. 8. In a preferred embodiment, the soundprocessor 810 acts to “spread” the sound image produced by the twoclosely spaced speakers 824, 825 by employing a cross-cancellationtechnique wherein a cancellation signal is derived, for example, fromthe difference between the left and right audio signals 811, 812. Theresulting difference signal is scaled, delayed (if necessary) andspectrally modified before being added to the left channel and, inopposite polarity, to the right channel. The spectral modification tothe difference channel preferably takes the form of a low-frequencyboost over a specified frequency range, in order to restore the correcttimbral balance after the differencing process which causes a loss ofbass when the low-frequency signals in each channel are similar. Theeffect of the sound processor 810 is to enlarge the perceived area andstability of the sound image produced by the speakers 324, 325, andprovide an effect of stereo sound despite the close proximity of thespeakers 324, 325.

FIG. 9-1 is a more detailed diagram of a sound processing system 900 inaccordance with various principles as disclosed herein, and as may beused, for example, in connection with the sound system 600 illustratedin FIG. 6, or more generally, in any sound system which utilizesmultiple audio channels to provide stereo source signals. In the soundprocessing system 900 of FIG. 9-1, a left audio signal 911 and rightaudio signal 912 are provided from an audio source, and may be fed toother speakers as well (not shown in FIG. 9-1). A difference between theleft audio signal 911 and right audio signal 912 is obtained by, e.g., asubtractor 940, and the difference signal 941 is fed to a spectralweighting filter 942, which applies a spectral weighting (and possibly again factor) to the difference signal 941. The characteristics of thespectral weighting filter 942 may vary depending upon a number offactors including the desired aural effect, the spacing of the speakers924, 925 with respect to one another, the taste of the listener, and soon. The output of the spectral weighting filter 942 may be provided to aphase equalizer 945, which compensates for the phase shifting caused bythe spectral weighting filter 942 (if non-linear).

In FIG. 9-1, the output of the phase equalizer 945 is provided to across-cancellation circuit 947. The cross-cancellation circuit 947 alsoreceives the left audio signal 911 and right audio signal 912, asadjusted by phase compensation circuits 955 and 956, respectively. Thephase compensation circuits 955, 956, which may be embodied as, e.g.,all-pass filters, preferably shift the phase of their respective inputsignals (i.e., left and right audio signals 911, 912) in a complementarymanner to the phase shifting performed by the phase equalizer 945 (incombination with the phase distortion caused by the spectral weightingfilter 924), such that the phase characteristic of the central channelis substantially 180° degrees out-of-phase with the phase characteristicof the left and right channels over the frequency band of interest. Thecross-cancellation circuit 947, which may include a pair of summingcircuits (one for each channel), then mixes the spectrally-weighted,phase-equalized difference signal, after adjusting for appropriatepolarity, with each of the phase-compensated left audio signal 911 andright audio signal 912. The perceived width of the soundstage producedby the pair of speakers 924, 925 can be adjusted by varying the gain ofthe difference signal path, and/or by modifying the shape of thespectral weighting filter 942.

FIG. 9-2 is a diagram of a sound processing system 900′ in generalaccordance with the principles and layout illustrated in FIG. 9-1,further showing typical examples of possible transfer functioncharacteristics for certain processing blocks. As with FIG. 9-1, in thesound processing system 900′ a left audio signal 911′ and a right audiosignal 912′ are provided from an audio source (not shown), and adifference signal 941′ is obtained representing the difference betweenthe left audio signal 911′ and the right audio signal 912′. Thedifference signal 941′ is fed to a spectral weighting filter 942′,which, in the instant example, applies a spectral weighting to thedifference signal 941′, the characteristics of which are graphicallyillustrated in the diagram of FIG. 9-2. A more detailed graph of thetransfer function characteristics (both gain and phase) of the spectralweighting filter 942′ in this example appears in FIG. 15-1. As showntherein, the spectral weighting filter 942′ is embodied as a first-ordershelf filter with a gain of 0 dB at low frequencies, and turn-overfrequencies at approximately 200 Hz and 2000 Hz. If desired, the gainapplied by gain/amplifier block 946′ can be integrated with the spectralweighting filter 942′, or the gain can be applied downstream asillustrated in FIG. 9-2. In any event, as previously noted, the turnoverfrequencies, amount of gain, slope, and other transfer functioncharacteristics may vary depending upon the desired application and/oroverall system characteristics.

A phase equalizer 945′ is provided in the center processing channel, andaddition phase compensation circuits 955′ and 956′ in the right and leftchannels, to ensure that the desired phase relationship is maintained,over the band of interest, between the center channel and the right andleft channels. As shown graphically in both FIG. 9-2 and in more detailin FIG. 15-1, the spectral weighting filter 942′ in the instant examplecauses a phase distortion over at least the 200 Hz to 2000 Hz range. Thephase equalizer 945′ provides no gain, but modifies the overallfrequency characteristic of the center channel. The phase compensationcircuits 955′ and 956′ likewise modify the phase characteristics of theleft and right channels, respectively. The phase compensation ispreferably selected, in the instant example, such that the phasecharacteristic of the center channel (that is, the combined phase effectof the spectral weighting filter 942′ and the phase equalizer 945′) isapproximately 180° out-of-phase with the phase characteristic of theleft and right channels, over the frequency band of interest (in thisexample, over the 200 Hz to 2000 Hz frequency band). At the same time,the phase characteristic of the left and right channels are preferablyremains the same, so that, among other things, monaural signals beingplayed over the left and right channels will have identical phaseprocessing on both channels (and thus maintain proper soundcharacteristics). Therefore, the phase compensation circuits 955′ and956′ preferably are configured to apply identical phase processing tothe left and right channels.

More detailed graphical examples of gain and phase transfer functions(with the gain being zero in this case when the components are embodiedas all-pass filters) are illustrated for the center channel phaseequalizer 945′ in FIG. 15-2 and for the left and right channel phasecompensation circuits 955′, 956′ in FIG. 15-3. In these examples, thephase equalizer 945′ is embodied as a second-order all-pass filter (withF=125 Hz and Q=0.12), and the phase compensators 955′ 956′ are eachembodied as second-order all-pass filters (with F=3200 Hz and Q=0.12). Ahigher Q value may be used to increase the steepness of the phasedrop-off, reducing the extent to which the center channel isout-of-phase with the left and right channels at low frequencies (thusminimizing the burden imposed upon the speakers 924′, 925′).

FIG. 11 illustrates another implementation of the sound system 900 shownin FIG. 9-1, where all-pass filters 1157, 1158 are used in phasecompensation blocks 1155 and 1156, respectively, to provide phaseequalization and/or compensation. In FIG. 11, elements labeled withreference numerals “11xx” generally correspond to their counterpartslabeled “9xx” in FIG. 9-1.

FIG. 10 is another diagram of a sound processing system 1000, inaccordance with the general principles explained with respect to FIGS. 3and 9, illustrating representative transfer functions according to anexemplary embodiment as described herein. In the sound processing system1000 shown in FIG. 10, input audio signals X1 and X2 (e.g., left andright audio signals) are processed along two parallel paths, and theresultants individually summed together and provided as output signalsY1 and Y2, respectively (which may be fed to a pair of speakers, e.g.,left and right speakers located in close proximity). A differencebetween the input audio signals X1 and X2 is obtained from a subtractor1040, which provides the resulting difference signal 1040 to aprocessing block 1060 having a transfer function −B. The first inputaudio signal X1 is also fed to a processing block 1055 having a transferfunction A, and the output of processing block 1055 is added togetherwith the output of processing block 1060 and fed as the first outputsignal Y1. Likewise, the second input audio signal X2 is fed to aprocessing block 1056 having a transfer function −A (i.e., the inverseof the transfer function A of processing block 1055), and the output ofprocessing block 1056 is inverted and added together with the invertedoutput of processing block 1060, then fed as the second output signalY2. The overall relationship between the inputs and the outputs of theFIG. 10 sound processing system 1000 can be expressed as:

${{A\left( {\begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix} + {B\begin{bmatrix}{- 1} & 1 \\1 & {- 1}\end{bmatrix}}} \right)}\begin{bmatrix}x_{1} \\x_{2}\end{bmatrix}} = \begin{bmatrix}y_{1} \\y_{2}\end{bmatrix}$

In a preferred embodiment, the transfer function −B of processing block1060 represents the combined transfer functions of a spectral weightingfilter of desired characteristics and a phase equalizer, such asillustrated by the difference path in the sound processing system 400 ofFIG. 4. Also in a preferred embodiment, the transfer functions A and −Aof processing blocks 1055 and 1056, respectively, each represent thetransfer function of a phase compensation network that performs acomplementary phase shifting to compensate for the phase effects causedby the processing block 1060. The polarities in FIG. 10 are selected sothat appropriate cross-cancellation will be attained.

In a preferred embodiment, input signals X1 and X2 represent theZ-transforms of the left and right audio channel inputs, and Y1 and Y2represent the corresponding Z-transforms of the left and right channeloutputs which feed the pair of speakers (e.g., left and right speakers)located in close proximity. The transfer functions A, −A, and B may berepresented in terms of z, and are determined in part by the samplingfrequency F_(S) associated with processing in the digital domain.According to a particular embodiment, blocks 1055 and 1056 are eachsecond-order all-pass filters with f=3200 Hertz, Q=0.12, and may, in oneexample, possess the following transfer function characteristics basedupon representative examples of the sampling frequency F_(S):

For F_(S)=48 KHz,

${A(z)} = \frac{{- 0.2578123} - {0.6780222\; z^{- 1}} + z^{- 2}}{1 - {0.6780222\; z^{- 1}} - {0.2578123\; z^{- 2}}}$

For F_(S)=44.1 KHz,

${A(z)} = \frac{0.2944196 - {0.633509\; z^{- 1}} + z^{- 2}}{1 - {0.633509\; z^{- 1}} - {0.2944196\; z^{- 2}}}$

For F_(S)=32 KHz,

${A(z)} = \frac{{- 0.4201395} - {0.469117\; z^{- 1}} + z^{- 2}}{1 - {0.469117\; z^{- 1}} - {0.4201395\; z^{- 2}}}$

In this particular embodiment, block 1060 may be a first-order shelfhaving a gain of 0 dB at low frequencies and turn-over frequencies of200 Hertz and 2 KHz in cascade with a second-order all pass filter, withf=125 Hz, Q=0.12, and may, in one example, possess the followingtransfer function characteristics based upon representative examples ofthe sampling frequency F_(S):

For F_(S)=48 KHz,

${B(z)} = {G \times \frac{0.1116288 - {0.085787\; 1z^{- 1}}}{1 - {0.9741583\; z^{- 1}}} \times \frac{0.8723543 - {1.872104\; z^{- 1}} + z^{- 2}}{1 - {1.872104\; z^{- 1}} + {0.8723543\; z^{- 2}}}}$

For F_(S)=44.1 KHz,

${B(z)} = {G \times \frac{0.1126427 - {0.0845478z^{- 1}}}{1 - {0.9719051\; z^{- 1}}} \times \frac{0.8618468 - {1.861552\; z^{- 1}} + z^{- 2}}{1 - {1.861552\; z^{- 1}} + {0.8618468\; z^{- 2}}}}$

For F_(S)=32 KHz,

${B(z)} = {G \times \frac{0.1173312 - {0.0788175z^{- 1}}}{1 - {0.9614863\; z^{- 1}}} \times \frac{0.814462 - {1.813915\; z^{- 1}} + z^{- 2}}{1 - {1.813915\; z^{- 1}} + {0.814462z^{- 2}}}}$

A gain factor may also be included in block 1060, or else may beprovided in the same path but as a different block or element. The gainmay be determined for a particular application by experimentation, butis generally expected to be optimal in the range of 10-15 dB. In oneembodiment, for example, the gain factor is 12 dB.

FIGS. 13 and 14 are graphs illustrating examples of frequency responseand phase transfer functions for a sound processing system in accordancewith FIG. 10 and having particular spectral weighting, equalization andphase compensation characteristics. FIG. 13 illustrates a frequencyresponse transfer function 1302 and phase transfer function 1305 for−B/A, which represents the transfer function of the difference channel(−B) and the first input channel (X1) with +12 dB of gain added. Asshown in FIG. 13, the frequency response transfer function 1302 exhibitsa relatively flat gain in a first region 1320 of bass frequencies (inthis example, up to about 200 Hertz), a decreasing gain in a secondregion 1321 of mid-range frequencies (in this example, from about 200Hertz to about 2 KHz), and then a relatively flat gain again in a thirdregion 1322 of high frequencies (in this example, above 2 KHz). Thephase response transfer function 1305 indicates that in the secondregion 1321 of mid-range frequencies (i.e., between about 200 Hertz and2 KHz) the output signal remains substantially in phase.

FIG. 14 illustrates a frequency response transfer function 1402 andphase transfer function 1405 for −B/−A, which represents the transferfunction of the difference channel (−B) and the first input channel (X2)with +12 dB of gain added. In FIG. 14, as with FIG. 13, the frequencyresponse transfer function 1402 exhibits a relatively flat gain in afirst region 1420 of bass frequencies (in this example, up to about 200Hertz), a decreasing gain in a second region 1421 of mid-rangefrequencies (in this example, from about 200 Hertz to about 2 KHz), andthen a relatively flat gain again in a third region 1422 of highfrequencies (in this example, above 2 KHz). The phase response transferfunction 1405 indicates that in the second region 1421 of mid-rangefrequencies (i.e., between about 200 Hertz and 2 KHz) the output signalis substantially inverted in phase (i.e., at 180 degrees).

As noted, the output signals Y1, Y2 are preferably provided to a pair ofspeakers located in close proximity. The transfer functions A, −A, and Bare examples selected for the situation where the speakers are locatedsubstantially adjacent to one another. However, benefits may be attainedin the system 1000 of FIG. 10, or other embodiments described herein,where the pair of speakers are not immediately adjacent, but arenevertheless in close proximity with one another.

FIG. 16 is a diagram of a sound processing system 1600 in accordancewith an alternative embodiment as described herein, employing a linearspectral weighting filter. In the sound processing system 1600 of FIG.16, a left audio signal 1611 and right audio signal 1612 are processedto derive a pair of processed audio signals 1648, 1649 which are appliedto a pair of closely spaced speakers 1624, 1625 (e.g., left and rightspeakers). The left and right audio signals 1611, 1612 are operated uponby a subtractor 1640, which outputs a difference signal 1641representing a difference between the left and right audio signals 1611,1612. The difference signal 1641 is fed to a spectral weighting filter1642 having a linear phase characteristic. The spectral weighting filter1642 may have frequency response characteristics in general accordance,for example, with the transfer function illustrated in FIG. 7A or 7B.Because the spectral weighting filter 1642 has a linear phasecharacteristic, phase equalization and compensation are not necessary.Therefore, the output of the spectral weighting filter 1642 may beprovided directly to a cross-cancellation circuit 1646, which then mixesthe spectrally weighted signal with each of the left and right audiochannels before applying them to the speakers 1624, 1625. To compensatefor the delay caused by the spectral weighting filter 1642, delaycomponents 1655 and 1656 may be added along the left and right channelpaths, respectively. The delay components 1655, 1656 preferably have adelay characteristic equal to the latency of the linear spectralweighting filter 1642.

The amount of cross-cancellation provided by the sound processing invarious embodiments generally determines the amount of “spread” of thesound image. If too much cross-cancellation is applied, then theresulting sound can seem clanky or echoey. If, on the other hand, toolittle cross-cancellation is applied, then the sound image may not besufficiently widened or stabilized.

The pair of speakers (e.g., speakers 824 and 825 in FIG. 8, or closelyspaced speakers in other embodiments described herein) which receive thesound processed information are preferably located immediately adjacentto one another; however, they may also be physically separated whilestill providing benefits of enlarged sound image, increased stability,and so on. Generally, the maximum acceptable separation of the pair ofspeakers can be determined by experimentation, but performance maygradually decline as the speakers are moved farther apart from oneanother. Preferably, the two speakers are placed no further apart than adistance that is comparable with the wavelength of the highest frequencythat is intended to be radiated by the speakers. For a maximum frequencyof 2 kHz, this separation would correspond to a center-to-center spacingof about 6 inches between the two speakers. However, ideally the twospeakers are placed immediately next to one another, in order to attainthe maximum benefit from the sound processing techniques as describedherein.

In various embodiments as described herein, improved sound qualityresults from a stereo sound image that has stability over a larger areathan would otherwise be experienced with, e.g., speakers spaced farapart without comparable sound processing. Consequently, the audioproduct (e.g., soundtrack) can be enjoyed with optimal or improved soundover a larger area, and by more listeners who are able to experienceimproved sound quality even when positioned elsewhere than the center ofthe speaker arrangement. Thus, for example, a home theater surroundsound system may be capable of providing quality sound to a greaternumber of listeners, not all of whom need to be positioned in the centerof the speaker arrangement in order to enjoy the playback of theparticular audio product.

In any of the foregoing embodiments, the audio product from which thevarious audio source signals are derived, before distribution to thevarious speakers or other system components, may comprise any audio workof any nature, such as, for example, a musical piece, a soundtrack to anaudio-visual work (such as a DVD or other digitally recorded medium), orany other source or content having an audio component. The audio productmay be read from a recorded medium, such as a DVD, cassette, compactdisc, CD-ROM, or else may be received wirelessly, in any availableformat, from a broadcast or point-to-point transmission. The audioproduct preferably has at least left channel and right channelinformation (whether or not encoded), but may also include additionalchannels and may, for example, be encoded in a surround sound or othermulti-channel format, such as Dolby-AC3, DTS, DVD-Audio, etc. The audioproduct may also comprise digital files stored, temporarily orpermanently, in any format used for audio playback, such as, forexample, an MP3 format or a digital multi-media format.

The various embodiments described herein can be implemented using eitherdigital or analog techniques, or any combination thereof. The term“circuit” as used herein is meant broadly to encompass analogcomponents, discrete digital components, microprocessor-based or digitalsignal processing (DSP), or any combination thereof. The invention isnot to be limited by the particular manner in which the operations ofthe various sound processing embodiments are carried out.

While examples have been provided herein of certain preferred orexemplary filter characteristics, transfer functions, and so on, it willbe understood that the particular characteristics of any of the systemcomponents may vary depending on the particular implementation, speakertype, relative speaker spacing, environmental conditions, and other suchfactors. Therefore, any specific characteristics provided herein aremeant to be illustrative and not limiting. Moreover, certain components,such as the spectral weighting filter described herein with respect tovarious embodiments, may be programmable so as to allow tailoring tosuit individual sound taste.

The spectral weighting filter in the various embodiments describedherein may provide spectral weighting over a band smaller or larger thanthe 200 Hertz to 2 KHz band. If the selected frequency band for spectralweighting is too large, then saturation may occur or clipping mayresult, while if the selected frequency band is too small, then thespreading effect may be inadequate. Also, if cross-cancellation is notmitigated at higher frequencies, as it is in the spectral weightingfilters illustrated in certain embodiments herein, then a comb filtereffect might result which will cause nulls at certain frequencies.Therefore, the spectral weighting frequency band, and the particularspectral weighting shape, is preferably selected to take account of thephysical limitations of the speakers and electronic components, as wellas the overall quality and effect of the speaker output.

While certain system components are described as being “connected” toone another, it should be understood that such language encompasses anytype of communication or transference of data, whether or not thecomponents are actually physically connected to one another, or elsewhether intervening elements are present. It will be understood thatvarious additional circuit or system components may be added withoutdeparting from teachings provided herein.

Certain embodiments of the invention may find application in a varietyof contexts other than home theater or surround sound systems. Forexample, implementations of the invention may, in some circumstances, beapplicable to personal computer systems (e.g., configured to play audiotracks, multi-media presentations, or video games with“three-dimensional” or multi-channel sound), automobile or vehicularaudio systems, portable stereos, televisions, radios, and any othercontext in which sound reproduction is desired. Certain embodiments mayfind particular utility in situations in which possible speakerlocations are limited and/or the maximum spacing between left and rightspeakers is severel limited, but where two adjacent or closely spacedspeakers could be achieved. For example, the pair of closely spaced leftand right speakers may be part of an integrated portable stereo unit, orelse may be located atop or beneath a computer monitor, etc.

In some embodiments, the pair of closely spaced speakers may be forcedto work harder than they would without cross-cancellation, because thecross-mixing of left and right signals requires that the speakersreproduce out-of-phase sound waves. To compensate for this effect, itmay, for example, be desirable in some embodiments to increase the sizeof the amplifier(s) feeding the audio signals to the pair of closelyspaced speakers.

While preferred embodiments of the invention have been described herein,many variations are possible which remain within the concept and scopeof the invention. Such variations would become clear to one of ordinaryskill in the art after inspection of the specification and the drawings.The invention therefore is not to be restricted except within the spiritand scope of any appended claims.

1. A sound reproduction system for a surround sound stereophonic system, comprising: a surround left speaker; a surround right speaker; a pair of surround back speakers located in close proximity and oriented in substantially the same direction, said surround back speakers comprising a surround back left speaker and a surround back right speaker; a surround left channel audio signal electrically connected to said surround left speaker; a surround right channel audio signal electrically connected to said surround right speaker; and a sound processor receiving as inputs said left channel audio signal and said right channel audio signal, said sound processor configured to generate a difference signal representing a difference between said surround left channel audio signal and said surround right channel audio signal, apply spectral weighting to said difference signal thereby generating a spectrally weighted signal, and cross-cancel said spectrally weighted signal with said surround left channel audio signal and said surround right channel audio signal to respectively generate a surround back left speaker signal and a surround back right speaker signal for said surround back left and right speakers. 